Due Date Calculator

How to use this calculator

1. Choose how you are dating the pregnancy: Last Period, Conception Date, Ultrasound or IVF Transfer Date. Only that method's fields appear.
2. Fill them in. Under Last Period you can also give your average cycle length if it is not the textbook 28 days.
3. Press Calculate Due Date to see your estimated due date, how far along you are today, and the whole timeline from your last period to full term.

The four ways to date a pregnancy

All four land on the same rule — 280 days from the last menstrual period — but they differ in how much guesswork stands between you and that date, so they are not equally certain.

• Last Period is the conventional route and the one prenatal care starts from, because the LMP is a date most people can name. It assumes ovulation around day 14, which is why a cycle that is not 28 days shifts the estimate.
• Conception Date counts 266 days forward instead of 280 — the same rule, started two weeks later. Useful if you know the date, which most people do not.
• Ultrasound is the most reliable early on, because the scan measures the pregnancy rather than inferring it from a cycle. Enter the scan date and the gestational age it reported, and the calculator works the LMP backwards from there.
• IVF Transfer Date is the most precise of the four. The embryo's age on transfer day is known exactly, so a 5-day transfer is due 261 days later and a 3-day transfer 263 — the older embryo is further along, so it arrives sooner.

Why cycle length matters

Naegele's rule assumes a 28-day cycle with ovulation on day 14. The stretch after ovulation is fairly fixed at about 14 days; it is the stretch before it that varies. So a 35-day cycle ovulates roughly a week later than a 28-day one, the pregnancy is a week younger than the calendar suggests, and the due date moves a week later. Give your average cycle length and the calculator makes that adjustment day for day.

What Naegele's rule is

Due dates are estimated with Naegele's rule, the standard method in prenatal care. It counts a full-term pregnancy as 280 days (40 weeks) from the first day of your last period. Dating from the LMP rather than conception is the convention because the LMP is a date most people can identify, whereas the exact day of conception usually is not.

The formula

The calculator works from your LMP:

• Due date = LMP + 280 days
• Estimated conception = LMP + 14 days (around ovulation in a typical cycle)

The 40 weeks are counted from the LMP, so the first two of them pass before conception actually occurs — an artefact of the dating convention, not an error.

A worked example

Suppose your last period began on January 1, 2026. Adding 280 days puts your estimated due date at October 8, 2026, and conception is estimated around January 15. If today were June 1, 2026 you would be 21 weeks and 4 days along — in the second trimester, and just past halfway to your due date.

The manual shortcut

You can approximate the same date in your head: take the first day of your last period, subtract three months, then add seven days (and a year). For a period starting March 10, that gives December 10 plus seven days — about December 17 the next year. Because months vary in length, the shortcut lands within a day or two of the exact 280-day count the calculator uses.

How far along you are

Gestational age is expressed in weeks and days — "14 weeks and 2 days," for example — and drives the timing of scans and tests. The calculator counts the days from your LMP to today, converts them to weeks and days, and places you in the first trimester (weeks 1–13), second (14–27) or third (28 onward).

Remember it is an estimate

Only about 4% of babies arrive on their exact due date; a full-term birth is anything from 37 to 42 weeks, so the due date marks the middle of a normal range rather than a deadline. A first-trimester ultrasound can refine the estimate and is generally more accurate than LMP dating if your cycle is irregular or its length is uncertain.

To follow your pregnancy week by week, use the pregnancy calculator. Your healthcare provider can confirm and adjust your due date based on your history and scans — the note under the calculator says the rest.